This invention relates to an infrared radiation detector. The detector has an envelope with a base. A pyroelectric element, having a body of pyroelectric material sandwiched between two electrodes, is present inside the envelope and is maintained spaced from the base by two support members. A circuit arrangement has at least one circuit element electrically connected to one of the electrodes of the pyroelectric element. The circuit arrangement is encapsulated within a package which is fastened to the base inside the envelope. The package has protruding conductive leads connected to the circuit arrangement. One of the support members is one of the conductive leads.
An infrared radiation detector employing a pyroelectric element can be used, for example, in an intruder alarm. It is because the intruder is a moving source of infrared radiation that his presence can be detected. The infrared radiation emitted by the intruder is converted by the detector into an electric signal which can be used to actuate an alarm.
It is usual to mount the pyroelectric element in an envelope. In order to isolate the detector from detrimental environmental effects and for optimum performance at low frequencies, the pyroelectric element is fixed spaced from a base part of the envelope. This minimizes heat transfer from other parts of the detector to the pyroelectric element, and it lengthens the thermal time constant.
A basic infrared radiation detector may comprise a single pyroelectric element sandwiched between two electrodes. As the pyroelectric element has a relatively high impedance, it is customary to include in the detector a circuit element which acts an amplifier stage or impedance matching stage. The circuit element may be a field effect transistor (hereinafter referred to as an FET). The gate of the FET is connected to one of the two electrodes of the pyroelectric element.
In more sophisticated detectors, the circuitry which is located in the proximity of and in the same envelope as the pyroelectric element may be more complex. For example, U.S. Pat. No. 4,198,564 (corresponding to United Kingdom patent GB No. 1,580,403) describes a detector which also includes a nonlinear network. The network protects the gate of the FET from excessive voltages resulting from large or sudden changes in input radiation.
As described in that patent, the pyroelectric element is maintained spaced from the base of the envelope by a U-shaped plinth member. The circuit elements which are located in the proximity of the pyroelectric element(s) are unencapsulated chips. The various electrical connections to these chips are made by bonded wires. Unfortunately, the technique of bonding wires to the relatively small contact areas of unencapsulated chips is both expensive and difficult to perform accurately, so that the yield of usefully bonded detectors can be undesirably low.
Another problem which results from the use of unencapsulated chips is the difficulty of performing certain desirable tests on the chip. For example, the measurements of low leakage current and low voltage noise in the circuit elements are difficult. These tests can, of course, be carried out after the chips have been included in the detector. However, then defective chips cannot be identified until the detector is virtually complete. It can be expensive to test the chips at this late stage because a defective chip then results in the rejection not only of the chip itself, but also of the whole detector of which that chip is a part.
These problems are overcome by the invention described and claimed in U.S. Pat. No. 4,489,238. In that case one of the electrodes of the pyroelectric element is connected to a circuit arrangement which is encapsulated within a so-called microminiature package. This package is mounted on the base inside the envelope and has protruding conductive leads connected to the circuit arrangement. The pyroelectric element is maintained spaced from the base by two support members, one of which is formed by one of the conductive leads of the package. The other support member is a separate pillar.